Integrated circuit designers have always sought the ideal semiconductor memory: a device that is randomly accessible, can be written or read very quickly, is non-volatile, but indefinitely alterable, consumes little power, and is scalable. The search for such devices has led to investigations into atomic-level properties of materials for switching and memory applications.
Studies have been conducted into electron spin transistors and memory components. Even in the absence of a magnetic field, some metal ions exhibit splitting of the electron spin energy levels. This is referred to as zero field splitting. Zero field splitting is different from Zeeman splitting (i.e., separation of the electron spin energy levels in the presence of an externally applied magnetic field). The difference being that some molecules may exhibit splitting of the electron energy levels at zero externally applied magnetic field, due in part, to the natural crystal fields present around a metal ion (in the case of molecules with transition metal ions) or to spin-spin coupling within a molecule or between molecules. Molecules with transition metals (e.g., Mn, V, Fe, Co, Cr, Ni, Cu, Zn, Cd, and others) are quite frequently paramagnetic and may have electron spin energy levels at zero magnetic field with an energy splitting between levels for which a spin transition is allowed that is within a range detectable with microwave radiation. For example, as shown in FIG. 1, Mn+3 ions have a spin system with an effective spin S=2, with a positive zero field splitting value. The inset portion of FIG. 1 is an expanded view of the Ms=±2 energy levels in the region of observed parallel mode electron paramagnetic resonance transitions (indicated by the double arrows). Analytical techniques, such as microwave spectroscopy or electron paramagnetic resonance (EPR) spectroscopy can identify molecular systems that exhibit zero field splitting properties.
Spin-spin interactions occur when there is at least one unpaired electron interacting with another unpaired electron (S greater than or equal to 1, where S is the effective spin). An example molecular system that could give rise to this situation includes a molecule containing Mn+3, which has a total spin S=2 (e.g., the molecule Mn(salen)). In this case, there are 4 unpaired electrons interacting with each other.
Microwave absorption spectroscopy has been used to identify atomic properties of chemical species. Microwave absorption has been shown to be a viable means of determining energy absorption at frequencies corresponding to the zero field splitting value of the absorbing material.
It would be advantageous to utilize the zero field splitting properties of ions as a memory device. It would be additionally advantageous if such a memory device was non-volatile or semi-volatile, operated at speeds necessary for present memory functions, and could be scaled to sub-micron sizes.